C# code to generate a unique variation from a provided filename. Works by appending or matching an iterator on the end of the file name.

gistfile1.cs

public static string GetUniqueFileName(string filePath, char directorySeparatorChar = '\\')
{
    if (String.IsNullOrWhiteSpace(filePath))
        return filePath;

    const string iteratorRegex = @"(-(?<iterator>[0-9]+)|\((?<iterator>[0-9]+)\))$";

    var fileName = Path.GetFileNameWithoutExtension(filePath) ?? string.Empty;
    var directoryName = Path.GetDirectoryName(filePath);
    var fileExtension = Path.GetExtension(filePath) ?? "";
    var match = Regex.Match(fileName, iteratorRegex);
    var iteratorValue = 1;

    if (match.Success) {
        iteratorValue = int.Parse(match.Groups["iterator"].Value) + 1;
        fileName = fileName.Substring(0, match.Groups["iterator"].Index - 1);
    }
    
    var newFileName = String.Concat(String.Format("{0}-{1}", fileName, iteratorValue), fileExtension).ToLowerInvariant();
    
    return String.Concat(directoryName, (!String.IsNullOrEmpty(directoryName) ? directorySeparatorChar.ToString() : ""), newFileName).Replace(Path.DirectorySeparatorChar, directorySeparatorChar);
}

Example inputs and outputs

• hello.txt ---> hello-1.txt
• hello-1.txt ---> hello-2.txt
• hello(2).txt ---> hello-3.txt